Printed circuit board and method of manufacturing the same

ABSTRACT

A plurality of conductor traces are formed on a porous base insulating layer made of porous ePTFE. Each conductor trace has a laminated structure of a seed layer and a conductor layer. A cover insulating layer is formed on the base insulating layer to cover each conductor trace. The ePTFE used as the porous base insulating layer has continuous pores. An average pore size of the ePTFE is not less than 0.05 μm and not more than 1.0 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printed circuit board and a method of manufacturing the same.

2. Description of the Background Art

Cells such as fuel cells or electronic equipment such as hard disk drives employ printed circuit boards as transmission paths of electrical signals among circuit elements. A resist layer having a given pattern is formed on a conductive layer on a base insulating layer during manufacture of the printed circuit board. In this state, an exposed region of the conductive layer is subjected to etching using an etching solution, thereby forming a given conductor pattern. After that, the resist layer is removed. A cover insulating layer is formed to cover the conductor pattern. In this manner, the printed circuit board having the desired conductor pattern can be manufactured by subjecting the conductive layer to the etching (see JP 2008-258482 A, for example).

A material for the base insulating layer of the printed circuit board is optimally selected depending on uses of the printed circuit board. For example, the use of a porous polyimide film as the base insulating layer has been proposed in JP 2000-319442 A and JP 2003-201362 A. This reduces a dielectric constant of the base insulating layer.

However, the polyimide film used as the base insulating layer of JP 2000-319442 A has fine continuous pores. Therefore, a chemical solution such as an etching solution enters the continuous pores of the polyimide film to flow to a back surface of the base insulating layer during manufacture of the printed circuit board of JP 2000-319442 A. Alternatively, the chemical solution remains in the continuous pores of the polyimide film.

Meanwhile, the polyimide film used as the base insulating layer of JP 2003-201362 A does not have the continuous pores, and therefore the chemical solution will not enter the porous material during manufacture of the printed circuit board of JP 2003-201362 A. However, it is considered that the dielectric constant of the base insulating layer of JP 2003-201362 A is larger than the dielectric constant of the base insulating layer of JP 2000-319442 A.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a printed circuit board and a method of manufacturing the same capable of preventing a solution from entering and remaining in a porous material, and reducing a dielectric constant of an insulating layer.

(1) According to an aspect of the present invention, a printed circuit board includes an insulating layer made of a porous material having continuous pores, and a conductor layer having a given pattern provided on the insulating layer, wherein the porous material includes porous polytetrafluoroethylene, and an average pore size of the continuous pores of the polytetrafluoroethylene is not more than 1.0 μm.

In the printed circuit board, the polytetrafluoroethylene of the insulating layer has the continuous pores whose average pore size is not more than 1.0 μm. This prevents the processing solution from entering the continuous pores of the polytetrafluoroethylene when the conductor layer having the given pattern is formed using the processing solution. Since the polytetrafluoroethylene has higher water repellency than other resin such as polyimide, the processing solution is prevented from remaining in the continuous pores even when the processing solution enters the continuous pores of the polytetrafluoroethylene.

The insulating layer is made of the porous polytetrafluoroethylene having the continuous pores. This results in a lower dielectric constant of the insulating layer than that of an insulating layer made of other resin such as polyimide.

This prevents the solution from entering and remaining in the porous material, and reduces the dielectric constant of the insulating layer.

(2) The average pore size of the continuous pores of the polytetrafluoroethylene may be not more than 0.2 μm. In this case, the processing solution is sufficiently prevented from entering the continuous pores of the polytetrafluoroethylene when the conductor layer having the given pattern is formed using the processing solution.

(3) The average pore size of the continuous pores of the polytetrafluoroethylene may be not more than 0.1 μm. In this case, the processing solution is further sufficiently prevented from entering the continuous pores of the polytetrafluoroethylene when the conductor layer having the given pattern is formed using the processing solution.

(4) The polytetrafluoroethylene may be expanded polytetrafluoroethylene. In this case, the porous polytetrafluoroethylene having the continuous pores can be easily formed.

(5) According to another aspect of the present invention, a method of manufacturing a printed circuit board includes the steps of preparing an insulating layer made of a porous material having continuous pores, and forming a conductor layer having a given pattern on the insulating layer using a processing solution, wherein the porous material includes porous polytetrafluoroethylene, and an average pore size of the continuous pores of the polytetrafluoroethylene is not more than 1.0 μm.

In the method of manufacturing the printed circuit board, the polytetrafluoroethylene of the insulating layer has the continuous pores whose average pore size is not more than 1.0 μm. This prevents the processing solution from entering the continuous pores of the polytetrafluoroethylene when the conductor layer having the given pattern is formed using the processing solution. Since the polytetrafluoroethylene has higher water repellency than other resin such as polyimide, the processing solution is prevented from remaining in the continuous pores even when the processing solution enters the continuous pores of the polytetrafluoroethylene.

The insulating layer is made of the porous polytetrafluoroethylene having the continuous pores. This results in a lower dielectric constant of the insulating layer than that of an insulating layer made of other resin such as polyimide.

This prevents the solution from entering and remaining in the porous material, and reduces the dielectric constant of the insulating layer.

Other features, elements, characteristics, and advantages of the present invention will become more apparent from the following description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a printed circuit board according to one embodiment of the present invention;

FIGS. 2 (a) to (c) are sectional views for illustrating steps in a first method of manufacturing the printed circuit board;

FIGS. 3 (a) to (c) are sectional views for illustrating steps in the first method of manufacturing the printed circuit board;

FIGS. 4 (a) to (d) are sectional views for illustrating steps in a second method of manufacturing the printed circuit board; and

FIGS. 5 (a) to (c) are sectional views for illustrating steps in the second method of manufacturing the printed circuit board.

DETAILED DESCRIPTION OF THE INVENTION

Description will be made of a flexible printed circuit board according to one embodiment of the present invention with reference to the drawings. In the following description, the flexible printed circuit board is abbreviated as the printed circuit board.

(1) Configuration of the Printed Circuit Board

FIG. 1 is a schematic sectional view of the printed circuit board according to the one embodiment of the present invention. As shown in FIG. 1, a plurality of conductor traces 2 are formed on a porous base insulating layer 1 made of ePTFE (expanded polytetrafluoroethylene). Porous PTFE having continuous pores can be easily formed by using the ePTFE.

Each conductor trace 2 is a wiring trace or a ground trace. Each conductor trace 2 has a laminated structure of a seed layer 2 a made of a laminated film of chromium and copper, for example, and a conductor layer 2 b made of copper, for example. A cover insulating layer 3 is formed on the base insulating layer 1 to cover each conductor trace 2.

The cover insulating layer 3 may be formed on the base insulating layer 1 to cover the entire conductor traces 2, or may be formed on the base insulating layer 1 to cover part of the conductor traces 2.

The ePTFE used as the porous base insulating layer 1 has continuous pores. The average pore size of the ePTFE is not less than 0.05 μm and not more than 1.0 μm, preferably not less than 0.05 μm and not more than 0.2 μm, and more preferably not less than 0.05 μm and not more than 0.1 μm.

The thickness of the porous base insulating layer 1 is not less than 1 μm and not more than 500 μm, preferably not less than 10 μm and not more than 200 μm, and more preferably not less than 10 μm and not more than 100 μm, for example. The porous base insulating layer 1 having the thickness of not more than 500 μm improves its flexibility, and the porous base insulating layer 1 having the thickness of not less than 1 μm improves its insulation characteristics.

The thickness of the seed layer 2 a is preferably not less than 0.05 μm and not more than 1 μm. The thickness of each of the conductor traces 2 is not less than 1 μm and not more than 100 μm, and preferably not less than 10 μm and not more than 50 μm, for example.

The thickness of the cover insulating layer 3 is not less than 1 μm and not more than 500 μm, preferably not less than 10 μm and not more than 200 μm, and more preferably not less than 10 μm and not more than 100 μm, for example. The cover insulating layer 3 having the thickness of not more than 500 μm improves its flexibility. The cover insulating layer 3 having the thickness of not less than 1 μm improves its insulation characteristics.

(2) First Method of Manufacturing the Printed Circuit Board

FIGS. 2 and 3 are sectional views for illustrating steps in a first method of manufacturing the printed circuit board 10. The example of FIG. 2 shows the method of manufacturing the printed circuit board 10 by a semi-additive method.

First, the seed layer 2 a made of the laminated film of chromium and copper, for example, is formed on the porous base insulating layer 1 made of ePTFE by a sputtering method, for example, as shown in FIG. 2 (a). The seed layer 2 a may be formed by electroless plating. After that, the seed layer 2 a is smoothed using a smoothing solution such as potassium persulfate (a smoothing step).

Next, a resist film is formed of a dry film resist or the like on the seed layer 2 a, and the resist film is exposed using a mask having a given pattern, and then developed using a developing solution such as sodium carbonate (a developing step). In this manner, a plating resist 21 is formed on the seed layer 2 a as shown in FIG. 2 (b).

The conductor layer 2 b made of copper, for example, is subsequently formed on the seed layer 2 a excluding a region of the plating resist 21 by electrolytic plating using a plating solution such as copper sulfate as shown in FIG. 2 (c) (a plating step).

Next, the plating resist 21 is removed by chemical etching (wet etching) or stripping using a stripping solution such as sodium hydrate as shown in FIG. 3 (a) (a stripping step). An exposed region of the seed layer 2 a is then removed by etching using an etching solution such as ferric chloride (an etching step). Thus, the plurality of conductor traces 2 each composed of the seed layer 2 a and the conductor layer 2 b are formed as shown in FIG. 3 (b). Each conductor trace 2 is a wiring trace or a ground trace, for example.

As shown in FIG. 3 (c), the cover insulating layer 3 made of polyimide, for example, is formed on the porous base insulating layer 1 to cover the plurality of conductor traces 2. Accordingly, the printed circuit board 10 shown in FIG. 1 is completed. The printed circuit board 10 is cleaned using pure water at a temperature of 20° C. or pure water at a temperature of 80° C. (a cleaning step) during or after the foregoing steps.

(3) Second Method of Manufacturing the Printed Circuit Board

FIGS. 4 and 5 are sectional views for illustrating steps in a second method of manufacturing the printed circuit board 10. The example of FIG. 4 shows the method of manufacturing the printed circuit board 10 by a subtractive method.

First, the seed layer 2 a made of the laminated film of chromium and copper, for example, is formed on the porous base insulating layer 1 made of ePTFE by the sputtering method, for example, as shown in FIG. 4 (a). The seed layer 2 a may be formed by electroless plating.

Next, the conductor layer 2 b made of copper, for example, is formed on the entire seed layer 2 a by electrolytic plating using a plating solution such as copper sulfate as shown in FIG. 4 (b) (a plating step). After that, the conductor layer 2 b is smoothed using a smoothing solution such as potassium persulfate (a smoothing step).

The resist film 22 is subsequently formed of a photosensitive dry film resist, for example, on the conductor layer 2 b as shown in FIG. 4 (c). The resist film 22 is exposed in the given pattern and then developed using the developing solution such as sodium carbonate or the like (a developing step), so that the etching resist 22 a is formed as shown in FIG. 4 (d).

Then, an exposed region of the conductor layer 2 b and an exposed region of the seed layer 2 a that are not covered with the etching resist 22 a are removed by etching using the etching solution such as ferric chloride (an etching step) as shown in FIG. 5 (a). After that, the etching resist 22 a is removed by the stripping solution such as sodium hydrate (a stripping step). Thus, the plurality of conductor traces 2 each composed of the seed layer 2 a and the conductor layer 2 b are formed as shown in FIG. 5 (b). Each conductor trace 2 is a wiring trace or a ground trace, for example.

As shown in FIG. 5 (c), the cover insulating layer 3 made of polyimide, for example, is formed on the porous base insulating layer 1 to cover the plurality of conductor traces 2. Accordingly, the printed circuit board 10 shown in FIG. 1 is completed. The printed circuit board 10 is cleaned using pure water at a temperature of 20° C. or pure water at a temperature of 80° C. (a cleaning step) during or after the foregoing steps.

(4) Other Embodiments

The material for the cover insulating layer 3 is not limited to polyimide. For example, ePTFE may be used similarly to the base insulating layer 1.

The seed layer 2 a may be a single layer film of chromium, for example, not limited to the laminated film of chromium and copper.

The material for the conductor layer 2 b is not limited to copper. For example, another metal material such as a copper alloy, gold and aluminum may be used.

The seed layer 2 a may not be formed when the conductor layer 2 b is formed by another method instead of the electrolytic plating.

(5) Effects

In the printed circuit board 10 according to the one embodiment of the present invention, the ePTFE of the base insulating layer 1 has the continuous pores whose average pore size is not more than 1.0 μm. This prevents the processing solution from entering the continuous pores of the ePTFE when the conductor traces 2 are formed using the processing solution. Since the ePTFE has higher water repellency than other resin such as polyimide, the processing solution is prevented from remaining in the continuous pores even when the processing solution enters the continuous pores of the ePTFE.

The relative dielectric constant of PTFE is 2.1, and the relative dielectric constant of polyimide is 3.5. The relative dielectric constant of the porous ePTFE having the continuous pores is even lower. This causes the dielectric constant of the base insulating layer 1 to be sufficiently lower than the dielectric constant of a base insulating layer made of other resin such as polyimide.

This prevents the processing solution from entering and remaining in the porous material, and reduces the dielectric constant of the base insulating layer 1. The average pore size of the continuous pores of the ePTFE is not more than 0.2 μm, thereby sufficiently preventing the processing solution from entering the continuous pores of the ePTFE. The average pore size of the continuous pores of the ePTFE is not more than 0.1 μm, thereby more sufficiently preventing the processing solution from entering the continuous pores of the ePTFE. The dielectric constant of the base insulating layer 1 is reduced, resulting in lower loss to be caused by dielectrics of the conductor traces 2.

The coefficient of friction of PTFE is smaller than the coefficient of friction of other resin such as polyimide, thus improving wear resistance of the base insulating layer 1. In addition, the printed circuit board 10 can be easily inserted into a narrow space, thus improving working efficiency.

Since the modulus of elasticity of PTFE is smaller than the modulus of elasticity of other resin such as polyimide, flexibility and bending characteristics of the base insulating layer 1 can be improved. The resistivity of PTFE (not less than 1×10¹⁸ Ω·cm) is higher than the resistivity of polyimide (1×10¹⁶ Ω·cm), thus eliminating the need to increase the thickness of the base insulating layer 1 in order to ensure insulation of the base insulating layer 1. This results in more improved flexibility and bending characteristics of the base insulating layer 1 than flexibility and bending characteristics of a base insulating layer made of other resin such as polyimide.

(6) Correspondences between Elements in the Claims and Parts in Embodiments

In the following paragraph, non-limiting examples of correspondences between various elements recited in the claims below and those described above with respect to various preferred embodiments of the present invention are explained.

In the above-described embodiment, the base insulating layer 1 is an example of an insulating layer, and the conductor trace 2 is an example of a conductor layer.

As each of various elements recited in the claims, various other elements having configurations or functions described in the claims can be also used.

(7) Inventive Examples and Comparative Examples

In inventive examples 1 to 5 and a comparative example 1, printed circuit boards 10 having porous base insulating layers 1 of different average pore sizes were manufactured based on the foregoing first method of manufacturing the printed circuit board. In inventive examples 6 to 10 and a comparative example 2, printed circuit boards 10 having porous base insulating layers 1 of different average pore sizes were manufactured based on the foregoing second method of manufacturing the printed circuit board.

PTFE porous films (TEMISH (registered trademark) NTF1121 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the inventive examples 1 and 6. The average pore size of the base insulating layer 1 was 0.1 μm, and the thickness of the base insulating layer 1 was 70 μm.

PTFE porous films (TEMISH NTF1122 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the inventive examples 2 and 7. The average pore size of the base insulating layer 1 was 0.2 μm, and the thickness of the base insulating layer 1 was 75 μm.

PTFE porous films (TEMISH NTF1125 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the inventive examples 3 and 8. The average pore size of the base insulating layer 1 was 0.5 μm, and the thickness of the base insulating layer 1 was 68 μm.

PTFE porous films (TEMISH NTF1128 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the inventive examples 4 and 9. The average pore size of the base insulating layer 1 was 0.8 μm, and the thickness of the base insulating layer 1 was 80 μm.

PTFE porous films (TEMISH NTF1131 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the inventive examples 5 and 10. The average pore size of the base insulating layer 1 was 1.0 μm, and the thickness of the base insulating layer 1 was 82 μm.

PTFE porous films (TEMISH NTF1133 by Nitto Denko Corporation) were used as the base insulating layers 1 in the printed circuit boards 10 of the comparative examples 1 and 2. The average pore size of the base insulating layer 1 was 3.0 μm, and the thickness of the base insulating layer 1 was 86 μm.

In each of the printed circuit boards 10 of the inventive examples 1 to 10 and the comparative examples 1, 2, the seed layer 2 a was made of the laminated film of chromium having a thickness of 50 nm and copper having a thickness of 100 nm. The thickness of the conductor layer 2 b was 12 μm.

In the first and second methods of manufacturing the printed circuit board, a processing solution was observed whether or not it flowed to the back surface of each of the printed circuit boards 10 of the inventive examples 1 to 10 and the comparative examples 1, 2 through the continuous pores of the base insulating layer 1 during each of the plating step, the smoothing step, the developing step, the etching step, the stripping step and the cleaning step.

The following processing solutions were used in the respective steps: an aqueous solution of copper sulfate in the plating step; an aqueous solution of potassium persulfate in the smoothing step; an aqueous solution of sodium carbonate in the developing step; an aqueous solution of ferric chloride in the etching step; an aqueous solution of sodium hydrate in the stripping step; and pure water at a temperature of 20° C. and pure water at a temperature of 80° C. in the cleaning step.

Table 1 shows results of testing whether the processing solutions flow to the base insulating layer 1 in the printed circuit boards 10 of the inventive examples 1 to 5 and the comparative example 1.

TABLE 1 PROCESSING SOLUTION INVENTIVE INVENTIVE INVENTIVE INVENTIVE INVENTIVE COMPARATIVE (AQUEOUS EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 1 SOLUTION) 0.1 μm 0.2 μm 0.5 μm 0.8 μm 1.0 μm 3.0 μm COPPER ◯ ◯ ◯ ◯ ◯ X SULFATE POTASSIUM ◯ ◯ ◯ ◯ ◯ X PERSULFATE SODIUM ◯ ◯ ◯ ◯ ◯ X CARBONATE FERRIC ◯ ◯ ◯ ◯ ◯ X CHLORIDE SODIUM ◯ ◯ ◯ ◯ ◯ X HYDRATE PURE WATER ◯ ◯ ◯ ◯ ◯ X (20° C.) PURE WATER ◯ ◯ ◯ ◯ ◯ X (80° C.)

As shown in Table 1, the processing solutions did not flow to the back surface through the continuous pores of the base insulating layer 1 in the printed circuit boards 10 of the inventive examples 1 to 5. Moreover, the processing solutions did not remain in the continuous pores. Meanwhile, the processing solutions flowed to the back surface through the continuous pores of the base insulating layer 1 in the printed circuit board 10 of the comparative example 1. In addition, the processing solutions remained in the continuous pores.

Table 2 shows results of testing whether the processing solutions flowed to the base insulating layer 1 in the printed circuit boards 10 of the inventive examples 6 to 10 and the comparative example 2.

TABLE 2 PROCESSING INVENTIVE SOLUTION INVENTIVE INVENTIVE INVENTIVE INVENTIVE EXAMPLE COMPARATIVE (AQUEOUS EXAMPLE 6 EXAMPLE 7 EXAMPLE 8 EXAMPLE 9 10 EXAMPLE 2 SOLUTION) 0.1 μm 0.2 μm 0.5 μm 0.8 μm 1.0 μm 3.0 μm COPPER ◯ ◯ ◯ ◯ ◯ X SULFATE POTASSIUM ◯ ◯ ◯ ◯ ◯ X PERSULFATE SODIUM ◯ ◯ ◯ ◯ ◯ X CARBONATE FERRIC ◯ ◯ ◯ ◯ ◯ X CHLORIDE SODIUM ◯ ◯ ◯ ◯ ◯ X HYDRATE PURE WATER ◯ ◯ ◯ ◯ ◯ X (20° C.) PURE WATER ◯ ◯ ◯ ◯ ◯ X (80° C.)

As shown in Table 2, the processing solutions did not flow to the back surface through the continuous pores of the base insulating layer 1 in the printed circuit boards 10 of the inventive examples 6 to 10. Moreover, the processing solutions did not remain in the continuous pores. Meanwhile, the processing solutions flowed to the back surface through the continuous pores of the base insulating layer 1 in the printed circuit board 10 of the comparative example 2. In addition, the processing solutions remained in the continuous pores.

The results of the inventive examples 1 to 10 and the comparative examples 1, 2 show that when the average pore size of the porous base insulating layer 1 is not more than 1.0 μm, the processing solutions can be prevented from flowing to the back surface through the continuous pores of the base insulating layer 1 and from remaining in the continuous pores, regardless of the method of manufacturing the printed circuit board 10.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. A printed circuit board comprising: an insulating layer made of a porous material having continuous pores; and a conductor layer having a given pattern provided on said insulating layer, wherein said porous material includes porous polytetrafluoroethylene, and an average pore size of the continuous pores of said polytetrafluoroethylene is not more than 1.0 μm.
 2. The printed circuit board according to claim 1, wherein the average pore size of the continuous pores of said polytetrafluoroethylene is not more than 0.2 μm.
 3. The printed circuit board according to claim 1, wherein the average pore size of the continuous pores of said polytetrafluoroethylene is not more than 0.1 μm.
 4. The printed circuit board according to claim 1, wherein said polytetrafluoroethylene is expanded polytetrafluoroethylene.
 5. A method of manufacturing a printed circuit board comprising the steps of: preparing an insulating layer made of a porous material having continuous pores; and forming a conductor layer having a given pattern on said insulating layer using a processing solution, wherein said porous material includes porous polytetrafluoroethylene, and an average pore size of the continuous pores of said polytetrafluoroethylene is not more than 1.0 μm. 